Calculating The Discrete Fourier Transform From Array

Hello.

I have a one dimension array where i store all the gray numbers that i get from all the pixels from a picture with this type 0.299 * R + 0.587 * G + 0.114 * B.(of course i have first calculated each R,G,B for every loop)

How can i calculate the frequency index real part and imaginary part from each value that i have stored in the one dimensional array and put this numbers (frequency index) into two different arrays, one for the real numbers and one for imaginary numbers??

Any ideas???